Pellet-Clad Interaction—Fuel Rod Failure
Strong pellet-clad interaction resulting in high stress in the cladding is one of the lead causes of preventable fuel failure in light water reactors. This type of reactor is fueled with multiple assemblies where the basic element of each assembly is a single fuel rod. A fuel rod is made of a long cylindrical tube of a zirconium alloy inside which a stack of uranium oxide pellets occupies the majority of the volume. Each of the fuel pellets is generally cylindrical in shape. Geometrical variations of the pellet structure include dishing the flat ends of the pellet, chamfering of the pellet edges, introducing a vertical hole in the pellet center, and varying the pellet height-to-diameter ratio. Material variations of the fuel pellets include the ceramic density relative to its maximum possible theoretical value, i.e. variation in the pellet material porosity. Pellet material variation includes the isotopic composition of the fissionable elements, i.e. uranium enrichment in the fissile isotope U-235 and the possibility of including plutonium oxide in the ceramic matrix. Another common variation of the pellet material is the addition of a neutron absorber for reactivity control, such as gadolinium or boron. Additives to the fuel pellets may include trace elements for modifying its microstructure such a chromium oxide doping.
The fuel tube, henceforth called clad or cladding, may also vary in its dimensions and material composition. The base material is zirconium alloyed with varying percentages of other elements, most common alloys being the well known Zircaloy 2 and Zircaloy 4. The clad structure is generally a thin-walled tube, where the diameter and the wall thickness-to-diameter ratio vary from design to design. In some fuel designs, the inner surface of the cladding tube is made of a so-called liner or barrier which is a thin layer of soft zirconium or a different zirconium-based alloy.
The cladding tube, sealed at one end, is loaded with fuel pellets and then pressurized with helium gas before it is sealed at the other end. While the stack of fuel pellets occupies the majority of the fuel rod volume, there is a free volume reserved for the collection of fission gasses mainly in the upper part, the so-called upper plenum, and optionally at the lower end. The upper plenum contains a spring to press the pellet stack down and maintain the fuel stack continuity. The inner diameter of the cladding is slightly larger than the outer diameter of the fuel pellets, leaving a space of the approximate size of 100 microns, henceforth called the gap or pellet-clad gap, which also varies depending on fuel design and manufacturing process.
Operational Effects on Fuel Rod
During reactor operation, the fuel rods are subjected to extreme conditions which must be understood to protect the integrity of the fuel rods. External to the fuel rods, the coolant is pressurized to approximately 7 MPa in the case of boiling water reactors (BWR) and 14 MPa in the case of pressurized water reactors (PWR). Under neutron irradiation, the nuclei of the heavy fissionable isotopes split into medium weight nuclei and emit additional neutrons to sustain the fission chain reaction while releasing energy. The heat generated in fuel rods per unit length is so high that steep temperature gradients between the center and surface of the fuel pellets develop which results in large internal stress gradients due to thermal expansion of the pellet material, with the effect that the ceramic pellet material is deformed and is often cracked. The multiple effects on the fuel pellets include volume swelling due to the fact that there are two fission product nuclei for each heavy isotope that undergoes fission. There is also the opposite effect of pellet volume reduction due to the collapse of the pores. The net volume change is initially negative, but a net volume increase occurs as the exposure to neutron flux continues and the so-called burnup increases. Some of the fission products are gaseous, such as Xe, and a fraction of these gases escape outside the pellet material microstructure into the free volume inside the fuel rod causing its pressure to increase. There are numerous chemical compounds that form from the fission products, and some of these have corrosive effects on the cladding material. Notable among the corrosive chemical agents is iodine, which forms a series of zirconium iodides, and the most corrosive of these iodides is ZrI4.
The irradiation effects on the clad include the microstructure defects that occur when the fast neutrons (energy >1 MeV) collide with the zirconium nuclei. The effect of the cumulative fast neutron flux, the so-called fluence, reduces the ductility of the cladding material, and also promotes the permanent deformation, the so-called irradiation-induced creep, when the material is subjected to mechanical stresses. Clad elongation under fast neutron irradiation also occurs without the influence of external stress, and so-called temperature-induced creep occurs under external stress without the presence of fast neutron irradiation. However, the processes involved are so complicated such that temperature-induced creep rate is also affected by the fast neutron fluence.
The mechanical stresses on the clad include the compressive component due to the external coolant pressure, which cause the so-called clad creep-down, which is accelerated under the influence of fast neutron irradiation, with the general effect of reducing the pellet-clad gap. A tensile stress component in the clad occurs when the gap vanishes as the pellet and clad come into direct contact, a condition called pellet-clad-mechanical interaction (PCMI) or pellet-clad interaction (PCI). The tensile stress that occurs due to PCMI is in the tangential direction, also called hoop stress. The magnitude of the tensile stress increases with the processes that cause the pellet volume to increase and decreases (stress relief) as the cladding material creeps out in response to the imposed stress. It should be noted that the zirconium alloys used for cladding material are highly anisotropic where the degree of anisotropy is dependent on the heat treatment of the cladding, i.e. being fully or partially re-crystallized under relatively high temperature as part of the manufacturing process. Large tensile stresses may result when a gap is initially closed before a large power increase causes the pellet temperature to increase and subsequently pellet volume increase due to thermal expansion. The amount of pellet volume increase is proportional to the power increase, and is larger when the pellet thermal conductivity is low, the latter being the case when the pellet temperature is high and when its burnup is high.
Stress-Corrosion-Cracking Failure Mode
The corrosive chemical interaction of iodine released from the irradiated pellets and zirconium cladding material is greatly accelerated under tensile stress. This process is called stress-corrosion-cracking (SCC). Thus, a strong PCI which generates high tensile stress is considered the direct cause for the corrosive cracking of the cladding material which may result in the breach of the cladding integrity and the release of radioactive material outside the cladding tube into the coolant. The prevention of this so-called PCI failure mode is the subject of this patent.